Electromagnetic soundings, including controlled source electromagnetic (“CSEM”) experiments, are conducted by transmitting an electromagnetic signal, typically a low-frequency periodic waveform, into the subsurface, and measuring the electromagnetic response. U.S. Pat. No. 6,603,313 to Srnka and U.S. Patent Publication No. 2003/0050759 (PCT Publication No. WO 03/025803) by Srnka and Carazzone disclose methods for using CSEM measurements to prospect for oil and gas, and to delineate known prospects.
As illustrated in FIG. 1, which shows an example of the equipment involved in performing a marine CSEM survey, one end of an antenna 105 is attached to a tow body 110, which is lowered to the desired water depth via a sub-sea tow cable 115. The tow body 110 is more than just an anchor point for the tow cable. It provides a place to contain the electrical components necessary for generating an electromagnetic source wave, and also may contain communication systems, positioning systems, speed of sound measuring devices, altimeters and the like, that are useful in surveying. A winch (not shown), attached to a surface vessel 120, controls the tow cable 115.
The antenna 105 is the transducer of electromagnetic fields used for the CSEM survey. Marine CSEM surveys typically use horizontal electric dipoles (HED) which may be made as follows. Two insulated wires are extended from two output terminals of a power generator capable of supplying electrical power with a desired frequency and waveform. The other end of each insulated wire is connected to an electrode. Alternatively, the insulation may be stripped from the end of the insulated wire and the bare wire becomes the electrode. The two electrodes are maintained a fixed distance apart. The dipole axis is maintained in a horizontal posture in the case of an HED. A current loop between the two electrodes is completed in a marine application by the water, the sea bottom, and possibly the air above the water.
The antenna 105 generates a time-varying electromagnetic field 130 which, in the example shown in FIG. 1, penetrates the sea bottom 125 to a formation 135. The time-varying electromagnetic field causes a time-varying current 140 to flow in the formation 135. The flow of the time-varying current 140 through the formation 135 induces another electromagnetic field 145. An array of sensors 150, typically located on the sea floor, receives, detects, and analyzes the electromagnetic field 145, and stores the resulting data or reports it to the surface for analysis. The characteristics of the received electromagnetic field 145 depend on the characteristics of the transmitted electromagnetic field 130, which are known to some degree, the characteristics of the formation 135, the characteristics of other subsurface features and formations, and noise. It is possible to determine some of the characteristics of the formation 135 by analyzing the received electromagnetic field 145 in this context.
A typical sensor 150, illustrated in FIG. 2, includes an electronics package 205 coupled to a ballast 210. The electronics package 205 includes four antennae 215 arranged approximately symmetrically around its periphery. The four antennae form two electric dipoles, as described above. One or more vertical antennae (not shown) may also be included to detect vertically oriented electromagnetic radiation. The antennae 215 receive the electromagnetic field 145 and equipment in the electronics package 205 detect, analyze and record data related to the phase and amplitude of the electromagnetic field 145. When sufficient data has been recorded or after a certain period of time has passed, the electronics package 205 releases from the ballast 210 and floats to the surface where it is recovered. On the surface, data is recovered from the electronics package 205. The data are analyzed.
A survey is composed of the data collected as the result of one or more traverses of the antenna 105 over an area of the seafloor containing one or more sensors 150. Typically, each of the traverses is known as a “towline.”
One of the most significant sources of noise for the CSEM application is magnetotelluric noise 155, shown on FIG. 1 as a set of arrowed lines. In addition, noise may stem from seafloor oceanic currents triggering mechanical vibration of the antennae (“strum”) or from imperfections in the sensor electronics.